a.k.a. permission-based, opt in
The act of explicitly requesting an e-mail distribution. For example, when you sign up to receive an e-mail newsletter, you are "opting" to receive it. "Double opt-in" means that a user has requested a subscription and then confirmed it by responding to an autoresponder's follow-up message. Web sites and other online entities must provide a way for users to opt-out-that's what separates legitimate e-mail from spam.
NetLingo Classification: Online Business